1. Field of the Invention
The invention relates to protective circuits and more specifically to circuits for protecting solenoids and other inductive devices from accidental overload due to rapid on-off cycling.
2. Description of the Prior Art
When an AC-powered solenoid is energized, a sudden in-rush of electrical current occurs which may typically be in the order of five times the normal holding current. In applications where the solenoid may be accidentally subjected to rapid on-off cycling, the resultant high RMS value of the driving current may be sufficient to cause overheating and failure of the solenoid. In a typical application, a pair of solenoids may be used to control hydraulic flow in a ship's steering system. Under certain weather and sea conditions, frequent correction of the ship's rudder angle may be required to maintain the ship on its desired course. Such maneuvers are accomplished by frequent on-off cycling of the solenoids and the consequent danger of system failure.
In the past, prevention of solenoid overload in marine steering systems has depended upon the skill of the helmsman in the case of manual steering systems or in permitting the rudder to deviate through sufficiently wide angles to reduce the frequency of on-off operations in automatic steering systems. In general, protection of solenoids has depended upon reducing the frequency of both on and off times. The present invention limits the minimum off time of solenoid operation without restricting the on time.